Vol. 10, 1977
tRNA-Protein Interactions
411
Approaches to Understanding the Mechanism of Specific Protein-Transfer RNA Interactions Paul R. Schimmel Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received February 17, 1977
Although the different tRNAs share much in common, there is a class of enzymes-the aminoacyl-tRNA synthetases-which accurately discriminate between various tRNA ~peciesrl-~For each amino acid there is at least one aminoacyl-tRNA synthetase which catalyzes the esterification of the amino acid to a ribose hydroxyl on the 3'-terminal adenosine of its cognate transfer RNA.1-4 This reaction must be done with absolute fidelity; if a synthetase were to attach its amino acid to a noncognate tRNA, the amino acid could then be incorporated into an incorrect position of a growing polypeptide chain, producing the same end result as a m~tation.~ A central question is the mechanism by which these enzymes specifically discriminate between various tRNA molecules. This question is particularly intriguing in view of the many common structural features shared by the various tRNA molecules.6 In recent years it has become clear that tRNA molecules may be designed as receptors for many different proteins, and also that they may serve functions other than those directly involving protein synthesis (see the introductory article of this issue6). Thus, the question of the mechanism by which proteins recognize specific tRNA molecules is of broad interest. It obviously presents a great challenge. The pursuit of this question has been given great impetus by the availability of a three-dimensional structural model for With this in mind, let us consider in detail some experimental approaches to understanding the mechanism of specific protein-tRNA complex formation. Studies of this kind have largely been carried out with the well-characterized synthetase-tRNA system^.^-^ The discussion which follows therefore concentrates on these systems. However, the approaches used and considerations involved are applicable to a broad range of protein-nucleic acid systems, many of which (such as chromatin) are more complex than the synthetase-tRNA systems. Stability of Synthetase-tRNA Complexes At the outset it is useful to consider thermodynamic features of synthetase-tRNA interactions. Although some studies have estimated the strengths of the interactions from steady-state kinetic data on the aminoacylation rea~tion,~-'l there have been few studies Paul Schimmel is Professor of Biochemistry and Biophysics at Massachusetts Institute of Technology. He received his A B . degree from Ohio Wesleyan University in 1982 and went on to do graduate studies at Tufts University School of Medicine, Massachusetts Institute of Technology, and Cornell University. He also carried out postdoctoral work at Stanford University. During the years of graduate and postdoctoral study he was associated with Gordon Hammes and Paul Flory. His research interests are on structure-function relationships of proteins, nucleic acids, and protein-nucleic acid complexes, with particular emphasis on transfer RNA and protein-transfer RNA interactions. He is the recipient of the 1978 American Chemical Society Pfizer Award in Enzyme Chemistry.
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Table I Association Constants for Cognate and Noncognate Enzyme-tRNA Complexes a t pH 5.5, 17 " C K, M" Enzyme tRNA Yeast ValRS
E. coli IleRS >
Val (yeast) Val (E. coli) Ile (E. coli) Phe (E. coli) Glu (E. coli) Ile (E. coli) Val (yeast) Phe (yeast, -Y)" Phe ( E . c o l i ) Tyr (E. c o l i ) Glu ( E . c o l i )
lo8
3.2 x 107 9.1 x l o 6 2.8 X l o 6
